On a grassy rooftop in midtown Manhattan, Mayor Michael Bloomberg announcedWednesday - Earth Day - that the city will try to pass legislation mandatingenvironmental changes that could reduce the city's total carbon output by 5percent. The proposed legislation is aimed at getting significant-sizedbuildings to meet tougher energy requirements anytime they renovate, to conductan energy audit every 10 years, and to make energy improvements that will payfor themselves within five years. According to the city, the energyimprovements could save landlords $750 million a year - assuming they upgradeeverything from their lighting systems to boilers.

New York's effort comes at a time when Congress is considering legislation tocap carbon emissions and then allow emitters to trade in carbon credits orallowances. The proposal, which had its first day of hearings on Wednesday, isconsidered to be President Obama's second greatest legislative priority afterhealthcare reform.

On Wednesday, Mr. Obama, visiting a wind-power plant in Newton, Iowa, called onAmerica to view a green emphasis as a way to lead the global economy in the21st century.

For its part, New York is looked to by other cities as an environmental leader.Its latest effort is being watched by other cities, and if they adopt suchplans, too, the impact could be big: The New York model requires olderbuildings, not just new ones, to become energy-efficient.

"This is a game-changer, because it's the first large-scale citywideprogram to cut emissions from existing buildings. It includes the privatesector, and it focuses on efficiency upgrades that have a financial costsavings," writes Fred Krupp, president of Environmental Defense Fund inNew York, in an e-mail. "And it does all that at the scale of New YorkCity - harnessing billions of square feet of real estate to help solve globalwarming."

According to the US Green Building Council, buildings in the US are responsiblefor 39 percent of the nation's carbon emissions and 32 percent of the energyusage. In New York, which has a low transportation carbon footprint because ofmass transit, buildings represent 80 percent of carbon emissions.

"Buildings must be a part of the solution to the climate crisis and theeconomic crisis," says Richard Fedrizzi, CEO of the council, which isknown for bestowing green status on buildings.

Despite the endorsement of some of America's most well-known environmentalgroups and some City Council leaders, it's not a given that the council willadopt the legislation.

Two years ago, the City Council considered requiring buildings to use biodieselto cut down on emissions. But that effort ran into opposition because there wasconcern that the biofuel would cause food prices to rise.

In addition, landlords in the city are starting to feel the effects of theeconomic downturn. Office vacancy rates are rising. Some prominent buildingowners, who purchased properties at the wrong time, are defaulting on payments.

Mayor Bloomberg's latest initiative is part of his plaNYC, an effort now 2years old to substantially reduce the city's carbon footprint. New York hasalready planted 174,189 trees (with a goal of 1 million), added 80.9 miles ofbike lanes, and launched 224 energy-efficiency projects for city buildings.

Among the major points of the new proposal:

A new energy code that must be met byexisting buildings of 50,000 square feet (approximately equivalent to 50apartment units) whenever they make reservations. This would go into effect in2010.

The requirement for an energy auditevery 10 years, which would go into effect in 2012.

A requirement for buildings to upgradetheir lighting by 2010.

To try to increase support for these moves, Bloomberg is characterizing them ascreating 19,000 construction jobs. At the announcement on Wednesday, a host ofunion leaders endorsed the plan.

The jobs would come as building owners replaced light fixtures, addedenergy-efficient faucets and showers, and cleaned and tuned their boilers. Some$16 million in federal funds would go toward training such workers.